Recent studies in the pulmonary circulation suggest a role for the endothelium-derived constricting factor, endothelin (ET), in mediating hypoxic vasoconstriction. However, the mechanism of action of ET in the pulmonary vasculature is unknown. Thus, the overall aim of this project is to characterize the mechanisms by which ET causes pulmonary vasoconstriction and to determine the effect of chronic hypoxia on this pathway. The Specific Aims of this proposal are: 1) characterize ET- induced vasoconstriction and Ca2+ response, 2) determine intra- and extracellular Ca2+ sources involved in ET-1-induced Ca2+ response, 3) determine the ionic mechanisms of ET-1-induced membrane depolarization and 4) determine if the vasoconstrictory and Ca2+ responses are altered after chronic hypoxia. For these studies, a unique combination of techniques will be used, including isolated, cannulated arteries, whole cell patch clamp and microfluorescence measurement, to study the contractile effects of ET on isolated perfused microvessels and changes in ion currents, membrane potential and intracellular Ca2+in pulmonary artery smooth muscle cells, both before and after exposure to chronic hypoxia. For patients with chronic obstructive pulmonary disease, the structural and functional changes induced by hypoxia correlate with the development of pulmonary hypertension, considerably worsening the prognosis for patients. In this study, we hope to produce new information about the mechanisms by which ET regulates pulmonary vascular tone and elucidate a possible pathophysiologic role for endogenous ET, providing clinically useful information for the prevention and treatment of chronic obstructive airway disease and subsequent pulmonary hypertension.